Hurricanes, Hurricane Harvey, Texas, natural disatser, preparedness, resilience | Homeland Security Newswire

FloodsHurricane Harvey: Most fatalities occurred outside flood zones

Published 20 April 2018

Researchers found that most Houston-area drowning deaths from Hurricane Harvey occurred outside the zones designated by government as being at higher risk of flooding: the 100- and 500-year floodplains. Drowning caused 80 percent of Harvey deaths, and the research showed that only 22 percent of fatalities in Houston’s 4,600-square-kilometre district, Harris County, occurred within the 100-year floodplain.

A Dutch-Texan team found that most Houston-area drowning deaths from Hurricane Harvey occurred outside the zones designated by government as being at higher risk of flooding: the 100- and 500-year floodplains. Harvey, one of the costliest storms in U.S. history, hit southeast Texas on 25 August 2017 causing unprecedented flooding and killing dozens. Researchers at Delft University of Technology in the Netherlands and Rice University in Texas published their results in the European Geosciences Union journal Natural Hazards and Earth System Sciences.

“It was surprising to me that so many fatalities occurred outside the flood zones,” says Sebastiaan Jonkman, a professor at Delft’s Hydraulic Engineering Department who led the new study.

The EGU notes that drowning caused 80 percent of Harvey deaths, and the research showed that only 22 percent of fatalities in Houston’s 4,600-square-kilometre district, Harris County, occurred within the 100-year floodplain, a mapped area that is used as the main indicator of flood risk in the U.S.

Flood zones, or floodplains, are low-lying areas surrounding rivers and streams that are subject to flooding. To assess flood risk for insurance purposes and to set development standards, U.S. authorities outline floodplains for 100- and 500-year floods. These events have a 1 percent probability (100-year flood) and a 0.2 percent probability (500-year) of occurring in any given year.

“Hurricane Harvey was much larger than a 100- or 500-year flood, so flooding outside of these boundaries was expected,” says Jonkman. Rainfall totals in the week after the hurricane made landfall were among the highest recorded in U.S. history, with over 1000 mm of rain falling in just three days in large parts of both Harris and surrounding counties. As a result, a report by Delft University found that “unprecedented flooding occurred over an area the size of the Netherlands.”

Nonetheless, it was surprising for the researchers to find that so many of Harvey’s fatalities happened outside the designated floodplains given that these